Or maybe Google accessed his (the leaker's) account? Both actions would be equally offensive but there's no smoking gun here. Maybe this guy left his email logged in when he took a bathroom break... maybe he "lost" his phone which had the email account in question setup.

A non Google email account can be accessed from anywhere. Maybe Gooogle installed a keylogger on his work computer? Maybe they broke into his house and accessed it what way? Nothing about that statement, or the article proves that Google read the reporter's email and not the leakers.